On 29 October 2026, the Unified Patent Court (UPC) held Enanta Pharmaceuticals’ patent EP 4 051 265 invalid and dismissed its Paxlovid patent action against Pfizer, according to Enanta’s Form 8-K filing.
Enanta has now lost in the US and, at first instance, in Europe. The US ruling shows how a gap in an early filing can decide a pharmaceutical patent fight, and the European ruling adds pressure on the same family.
What happened in the Paxlovid patent case
Enanta announced on 20 August 2025 that it had sued Pfizer at the UPC, on the day the grant of the European patent was published. The action covered the 18 UPC member states then participating. It sought a finding of liability over Paxlovid, Pfizer’s oral COVID-19 antiviral, with damages to be decided later.
Pfizer answered with a counterclaim for revocation. The court heard the infringement action and the counterclaim together on 29 September 2026, and gave its decision a month later.
In its 8-K, Enanta reported that the court found the patent invalid and treated the infringement claim as unfounded for that reason. The filing does not set out the grounds. The written decision could not be checked on the UPC website at the time of writing.
According to ip fray, the decision came from the Munich Local Division. The same outlet reports that a preliminary view from the European Patent Office (EPO), where the patent is under opposition, is also unfavourable to Enanta. Both points are reported and have not been confirmed against the decision or the EPO file.
Enanta did not say in the filing whether it would appeal. An appeal would go to the UPC Court of Appeal in Luxembourg.
Not the first loss, and not the only COVID patent fight
The European result follows the US case on the counterpart patent. On 23 June 2026, the Federal Circuit affirmed a December 2024 summary judgment from the District of Massachusetts that Enanta’s US patent 11,358,953 was invalid.
The issue was priority. Enanta’s provisional application of July 2020 described compounds with a C2 to C12 alkyl group. The patent claims extended that range to C1 to C12, which covers the C1 group found in nirmatrelvir, the active ingredient of Paxlovid.
Judge Lourie, writing for the panel, held that the provisional did not disclose the C1 group. Without the earlier date, Pfizer’s public presentation of nirmatrelvir on 6 April 2021 anticipated the claims.
A parallel vaccine dispute shows that these fights are not one-way. On 1 August 2025, the Court of Appeal of England and Wales upheld a ruling that Moderna’s patent EP 3 590 949 was valid and infringed by the Pfizer and BioNTech vaccine. In that case the patent withstood the prior-art attack.
The wider picture in pharmaceutical claim scope
3 earlier decisions frame the same pressure from different angles. In Amgen v Sanofi, decided on 18 May 2023, the US Supreme Court held unanimously that broad functional antibody claims were invalid because the specification did not enable their full scope.
In Europe, the EPO Enlarged Board of Appeal ruled in G 1/22 and G 2/22 on 10 October 2023 that entitlement to priority is subject to a rebuttable presumption. That makes formal challenges harder. It covers who may claim priority, not whether the earlier filing discloses the same invention, which is assessed separately.
The UPC is not simply hostile to drug patents. On 25 November 2025, its Court of Appeal overturned a first-instance revocation of Amgen’s antibody patent EP 3 666 797, directed at the cholesterol target PCSK9, and held it valid against Sanofi and Regeneron. The court asked whether the skilled person would have pursued the claimed approach with a reasonable expectation of success.
Taken together, the cases show courts in both systems testing claims closely against the disclosure and the prior art, with outcomes that depend on the facts of each filing.
What it means for drug developers and their patent teams
For applicants, the first filing is the asset. A provisional or priority application written at speed should still state every substituent range, value and embodiment that later claims may need. A one-carbon gap in a Markush-type group cost Enanta its US priority date, a risk discussed in Synoptic IP’s guide to Markush structure searching.
For implementers facing assertion, the Enanta cases show the value of checking priority entitlement early. An invalidity search that maps each claim element to the priority document, and then searches the gap period, can surface art such as a competitor’s own public disclosure.
For patent owners asserting in Europe, the UPC lets a defendant attack validity and infringement in one proceeding. Here the action moved from filing to decision in about 14 months.
Synoptic IP’s view
Synoptic IP’s view is that the US Enanta ruling is less about Paxlovid than about drafting discipline at the priority stage. The court decided the case on what the July 2020 provisional said, and the result turned on a single substituent range.
Fast-moving fields, such as pandemic antivirals, invite fast filings. The US case shows the cost when those filings omit what later claims need. The Moderna and Amgen decisions show the other side: patents that withstand prior-art attacks can survive determined challenges in the UK and at the UPC.
The strongest counter-argument is that the UPC grounds are not yet public. The Munich decision may rest on inventive step or other reasons unrelated to priority, and the EPO opposition is reportedly still pending. If so, the European result says more about the prior art than about drafting.
Even then, the US ruling stands on its own facts. The practical lesson for applicants does not depend on how the UPC reasoned.
Teams reviewing a drug patent’s priority chain or preparing a validity challenge can start with our patent invalidity search or a chemical structure search of Markush-type claims.
This article is commentary and opinion based on public reports as of 2 November 2026 and is not legal advice.